The present invention relates to a set of progressive multifocal ophthalmic lenses; it also relates to a method for determining an ergorama for a set of progressive multifocal ophthalmic lenses, said ergorama providing an association, for each lens, between a point towards which the glance is directed and each direction of glance in the conditions under which the lens is actually worn. Finally, it relates to a method for defining a progressive ophthalmic lens.
Progressive multifocal ophthalmic lenses are now well-known. They are used for correcting long-sightedness and allow wearers of spectacles to look at objects over a large range of distances without needing to take their glasses off. Such lenses typically comprise a far vision region situated in the upper portion of the lens, and a near vision region situated in the lower part of the lens and an intermediate vision region linking the near vision region and the far vision region, together with a main meridian of progression which passes through these three regions.
French Patent 2,699,294 discusses, in its preamble, the various elements of such a progressive multifocal ophthalmic lens together with the work carried out by the present applicant to improve the comfort of wearers of such lenses. Reference should be made to that document for more information on these various points.
The applicant also proposed, for example in U.S. Pat. No. 5,270,745 or 5,272,495 to cause the meridian to vary, and notably to off-center it towards a near vision control point, as a function of power addition and ametropia.
Applicant has also proposed, in order to better satisfy the viewing requirements of long-sighted people and improve progressive multifocal lens comfort, various improvements (French Patents 2,683,642, 2,699, 294, 2,704,327).
Usually, these progressive multifocal lenses comprise a front aspherical face which is the face that faces away from the wearer of the spectacles, and a rear spherical or toroidal face directed towards the wearer of the spectacles. This spherical or toroidal face makes it possible to adapt the lens to the user's ametropia, meaning that a progressive multifocal lens is only generally defined by its aspherical surface. As is well known, such a aspherical surface is generally defined by the height or altitude of all points thereon. Parameters are also used consisting of maximum and minimum curvature at each point, or, more frequently, their half-sum and their difference. The half-sum and difference, multiplied by a factor n-1, n being the refractive index of the lens material, are called mean sphere or power, and cylinder.
Families of progressive multifocal lenses can be defined, each lens in a family being characterised by a power addition, corresponding to a variation in power between the far vision region and the near vision region. More precisely, power addition, referred to as A, corresponds to the variation in power between a point L in the far vision region and a point P in the near vision region, which are respectively referred to as the far vision control point and near vision control point, and which represent points where the glance intersects the surface of a lens for viewing to infinity and for reading vision.
Within the same family of lenses, power addition varies from one lens to another in the family between a minimum and maximum value of power addition. Usually, the minimum and maximum power addition values are respectively 0.75 diopters and 3.5 diopters, and power addition varies by 0.25 diopters steps from one lens to the next one in the family.
Lenses having the same power addition differ by their value of mean sphere at a reference point, also known as the base. One can for example decide to measure the base at the far vision control point L.
Thus, by choosing pairs, (power addition, base) a set of front aspherical faces for progressive multifocal lenses are defined. Usually, one can thus define five values for the base and 12 values for power addition, giving a total of 60 front faces. In each one of the bases, optimization is carried out for a given power.
Using, with one of these front faces, a rear face which is spherical or toroidal and near to the rear face used for optimization, makes it possible to cover all of the requirements of progressive multifocal lens wearers. This known method makes it possible, starting from semi-finished lenses of which only the front faces is shaped, to prepare lenses suited to each wearer, by simply machining a spherical or toroidal rear face.
This method suffers however from the disadvantage of only being an approximation; consequently, the results obtained with a rear face different from the one used for optimization are not as good as those corresponding to the rear face used for the optimization.
U.S. Pat. No. 5,444,503 discloses a progressive multifocal lens in which the rear face is adapted to each wearer, and is constituted by an aspherical surface. This aspherical surface is a not multifocal and appears to be calculated so as to provide the optical power necessary at certain reference points. In that Patent, it is considered the solution would make it possible to overcome the defects arising from replacing the rear space used for optimization by a rear face approximating it.
This solution has the disadvantage of considerably complicating lens manufacture: it implies measurement of the position of the lenses on the the wearer, followed by determination, and machining, of an aspherical rear face.